Opto-electronic devices that make use of organic materials are becoming increasingly desirable for a number of reasons. Many of the materials used to make such devices are relatively inexpensive, so organic opto-electronic devices have the potential for cost advantages over inorganic devices. In addition, the inherent properties of organic materials, such as their flexibility, may make them well suited for particular applications such as fabrication on a flexible substrate. Examples of organic opto-electronic devices include organic light emitting diodes/devices (OLEDs), organic phototransistors, organic photovoltaic cells, and organic photodetectors. For OLEDs, the organic materials may have performance advantages over conventional materials. For example, the wavelength at which an organic emissive layer emits light may generally be readily tuned with appropriate dopants.
OLEDs make use of thin organic films that emit light when voltage is applied across the device. OLEDs are becoming an increasingly interesting technology for use in applications such as flat panel displays, illumination, and backlighting. Several OLED materials and configurations are described in U.S. Pat. Nos. 5,844,363, 6,303,238, and 5,707,745, which are incorporated herein by reference in their entirety.
One application for phosphorescent emissive molecules is a full color display. Industry standards for such a display call for pixels adapted to emit particular colors, referred to as “saturated” colors. In particular, these standards call for saturated red, green, and blue pixels. Alternatively the OLED can be designed to emit white light. In conventional liquid crystal displays emission from a white backlight is filtered using absorption filters to produce red, green and blue emission. The same technique can also be used with OLEDs. The white OLED can be either a single EML device or a stack structure. Color may be measured using CIE coordinates, which are well known to the art.
One example of a green emissive molecule is tris(2-phenylpyridine) iridium, denoted Ir(ppy)3, which has the following structure:

As used herein, the term “organic” includes polymeric materials as well as small molecule organic materials that may be used to fabricate organic opto-electronic devices. “Small molecule” refers to any organic material that is not a polymer, and “small molecules” may actually be quite large. Small molecules may include repeat units in some circumstances. For example, using a long chain alkyl group as a substituent does not remove a molecule from the “small molecule” class. Small molecules may also be incorporated into polymers, for example as a pendent group on a polymer backbone or as a part of the backbone. Small molecules may also serve as the core moiety of a dendrimer, which consists of a series of chemical shells built on the core moiety. The core moiety of a dendrimer may be a fluorescent or phosphorescent small molecule emitter. A dendrimer may be a “small molecule,” and it is believed that all dendrimers currently used in the field of OLEDs are small molecules.
As used herein, “top” means furthest away from the substrate, while “bottom” means closest to the substrate. Where a first layer is described as “disposed over” a second layer, the first layer is disposed further away from substrate. There may be other layers between the first and second layer, unless it is specified that the first layer is “in contact with” the second layer. For example, a cathode may be described as “disposed over” an anode, even though there are various organic layers in between.
As used herein, “solution processible” means capable of being dissolved, dispersed, or transported in and/or deposited from a liquid medium, either in solution or suspension form.
A ligand may be referred to as “photoactive” when it is believed that the ligand directly contributes to the photoactive properties of an emissive material. A ligand may be referred to as “ancillary” when it is believed that the ligand does not contribute to the photoactive properties of an emissive material, although an ancillary ligand may alter the properties of a photoactive ligand.
As used herein, and as would be generally understood by one skilled in the art, a first “Highest Occupied Molecular Orbital” (HOMO) or “Lowest Unoccupied Molecular Orbital” (LUMO) energy level is “greater than” or “higher than” a second HOMO or LUMO energy level if the first energy level is closer to the vacuum energy level. Since ionization potentials (IP) are measured as a negative energy relative to a vacuum level, a higher HOMO energy level corresponds to an IP having a smaller absolute value (an IP that is less negative). Similarly, a higher LUMO energy level corresponds to an electron affinity (EA) having a smaller absolute value (an EA that is less negative). On a conventional energy level diagram, with the vacuum level at the top, the LUMO energy level of a material is higher than the HOMO energy level of the same material. A “higher” HOMO or LUMO energy level appears closer to the top of such a diagram than a “lower” HOMO or LUMO energy level.
As used herein, and as would be generally understood by one skilled in the art, a first work function is “greater than” or “higher than” a second work function if the first work function has a higher absolute value. Because work functions are generally measured as negative numbers relative to vacuum level, this means that a “higher” work function is more negative. On a conventional energy level diagram, with the vacuum level at the top, a “higher” work function is illustrated as further away from the vacuum level in the downward direction. Thus, the definitions of HOMO and LUMO energy levels follow a different convention than work functions.
More details on OLEDs, and the definitions described above, can be found in U.S. Pat. No. 7,279,704, which is incorporated herein by reference in its entirety.
Industry standards for such a display call for pixels adapted to emit particular colors, referred to as “saturated” colors. In particular, these standards call for saturated red, green, and blue pixels. Color may be measured using CIE coordinates well known and recognized in the art of lighting and displays. Alternatively, an OLED can be designed to emit white light, that is, light from two or more emitter dopants, e.g., three different color emitter dopants, combine to provide across most of the visible spectrum. One possible objective of a white OLED would be to provide light that mimics sunlight minus much of the strong UV. Accordingly, one can foresee a low cost, low power, high efficiency alternative to traditional incandescent lighting devices as well as light from LEDs. The white OLED can be either a single EML device or a stack structure. See, for example, U.S. Pat. Nos. 9,331,299; 9,559,151; 9,577,221; and 9,655,199, each of which is assigned to Universal Display Corporation (UDC), and each of which is incorporated by reference in its entirety.
Phosphorescent organic light-emitting devices (PHOLEDs) can achieve 100% internal quantum efficiency, however a considerable amount of this generated light is lost within the device structure due to the excitation of substrate, waveguide, and surface plasmon polariton (SPP) modes. Substrate modes can be efficiently outcoupled by structuring the air-substrate interface such as by using microlens arrays. See, Forrest, S. R.; et al., “Organic Light Emitting Devices with Enhanced Outcoupling via Microlenses Fabricated by Imprint Lithography”, in J. Appl. Phys. 2006, 100 (7), 73106. The major loss channels for trapped light beyond the modes trapped in the substrate are waveguide modes and SPPs. The optical power trapped inside the active region excites two different modes: the waveguide mode (power guided within the organic layer and transparent anode), and SPPs consisting of power confined at the metal/organic interface. Waveguide modes propagate tens of micrometers and can be efficiently scattered out of the device with appropriate outcoupling structures. In contrast, SPP modes are excited primarily in the metal cathode, propagate only a few micrometers, and dissipate before scattering, and therefore, results in an important and unsolved optical loss mechanism.
Loss of light output due to waveguide and SPP modes, which is typically greater than 50% in conventional OLEDs, remains a significant hurdle. Several methods such as sub-anode structures (see, Forrest, S. R.; et al., “Enhancing Waveguided Light Extraction in Organic LEDs Using an Ultra-Low-Index Grid”, in Opt. Lett. 2010, 35 (7), 1052-1054), high refractive index substrates (see, Mladenovski, S.; et al.; “Exceptionally Efficient Organic Light Emitting Devices Using High Refractive Index Substrates” in Opt. Express 2009, 17 (9), 7562-7570), scattering layers, (see, Koh, T. W.; et al., “Enhanced Outcoupling in Organic Light-Emitting Diodes via a High-Index Contrast Scattering Layer”, in ACS Photonics 2015, 2 (9), 1366-1372), corrugated structures, (see, Ou, Q.; et al., “Extremely Efficient White Organic Light-Emitting Diodes for General Lighting” in Adv. Funct. Mater. 2014, 24 (46), 7249-7256), Bragg scatterers, and microcavities (see, Wang, Z. B.; et al., “Unlocking the Full Potential of Organic Light-Emitting Diodes on Flexible Plastic” in Nat. Photonics 2011, 5 (12), 753-757) have been demonstrated to overcome these losses, although near-field coupling into SPP modes by the metal electrode is more difficult to avoid.
In particular, top-emitting OLEDs efficiently excite both waveguide and SPP modes due to the strong optical cavity formed between the high reflectivity semitransparent top electrode and the thick metallic bottom electrode. Several strategies such as thick electron transport layers, metallic grids, and periodically corrugated metal electrodes can help enhance light output. However, these methods are often wavelength and viewing-angle dependent, and they can be challenging to apply over large substrate areas.